Problem 15.Problem 16.15 (a) Rank-order the photons according to their wavelengths,
frequencies, and energies. If two are equal, say so. Explain all your
answers.
(b) Photon 3 was emitted by a xenon atom going from its second-
lowest-energy state to its lowest-energy state. Which of photons 1,
2, and 4 are capable of exciting a xenon atom from its lowest-energy
state to its second-lowest-energy state? Explain.
16 The figures show the wavefunction of an electron as a function
of position. Which one could represent an electron speeding up as
it moves to the right? Explain.
17 The beam of a 100 W overhead projector covers an area
of 1 m×1 m when it hits the screen 3 m away. Estimate the
number of photons that are in flight at any given time. (Since this
is only an estimate, we can ignore the fact that the beam is not
parallel.)√18 In the photoelectric effect, electrons are observed with virtu-
ally no time delay (∼10 ns), even when the light source is very weak.
(A weak light source does however only produce a small number of
ejected electrons.) The purpose of this problem is to show that the
lack of a significant time delay contradicted the classical wave the-
ory of light, so throughout this problem you should put yourself in
the shoes of a classical physicist and pretend you don’t know about
photons at all. At that time, it was thought that the electron might
have a radius on the order of 10−^15 m. (Recent experiments have
shown that if the electron has any finite size at all, it is far smaller.)
(a) Estimate the power that would be soaked up by a single electron
in a beam of light with an intensity of 1 mW/m^2.√(b) The energy,Es, required for the electron to escape through the
surface of the cathode is on the order of 10−^19 J. Find how long it
would take the electron to absorb this amount of energy, and explain
why your result constitutes strong evidence that there is something
wrong with the classical theory.
√Problems 943